1. Field of the Invention
The present invention is directed to a method and apparatus for resolving optical emissions and absorptions, and more particularly to a method and apparatus for resolving optical emissions and absorptions in at least two dimensions.
2. Discussion of the Background
Recently, the use of optical diagnostics in plasma processing tools has seen a significant increase. Optical diagnostics provide the benefits of real-time signal acquisition along with being inherently non-intrusive. Known systems using optical diagnostics, such as optical emission spectroscopy, acquire signals from only a single line of sight in space at a time, typically via an optical fiber feed. At one end, light emitted from the plasma passes through a small aperture (or iris) located outside an optical vacuum window and it is focussed onto one end of the optical fiber via a focusing lens. The opposite end of the optical fiber is generally optically connected to the input of a spectrometer, wherein the light spectrum may be dispersed via a grating and the incremental wavelength spectrum recorded using a photo-detector. In such a system, acquiring a signal from another line of sight in space requires repositioning of the optical system, meaning that the measurement at the next point in space is not done at the same time as for the previous line(s) of sight.
Other known optical systems use multiple fixed optical fiber feeds. The use of multiple feeds allows the estimation of the variation, at the same instant of time, of the measured property within some region of the plasma, but such systems do not support full multi-dimensional distributions of measured plasma properties since they suffer from the “one optical fiber channel per measurement line of sight” limitation. Moreover, such systems also suffer from the fact that the optical emission or absorption they intend to measure is, in actuality, the integrated light emitted or absorbed along the line of sight which falls within the field of view of the optical apparatus.
One method and device for detecting the end point of a plasma process is disclosed in U.S. Pat. No. 5,980,767 (hereinafter “the '767 patent”), assigned to the assignee of the present invention. FIG. 1 of the '767 patent is reproduced as FIG. 1 of the present application. In that figure, a single detector 22 is used to analyze the condition of the plasma.